Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, such as a copying machine or a printer.
Description of the Related Art
An electrophotographic image forming apparatus has a function of measuring a measurement image formed on an image bearing member with an optical sensor including a light emitting element and a light receiving element, and of appropriately adjusting density of an image to be formed by the image forming apparatus based on a measurement result of the measurement image. The optical sensor is configured to regularly adjust a driving current of the light emitting element with respect to a toner soil and a change with time of the light emitting element such that a detected value, which is the measurement result, is controlled to be kept constant within a predetermined range (Japanese Patent Application Laid-open No. 2008-209821). When a toner adhesion amount (image density) of the measurement image on the image bearing member is changed, an amount of reflected light from the image bearing member is changed. The optical sensor is configured to measure the change in amount of reflected light from the image bearing member, which is changed with the image density of the measurement image. In order to adjust an amount of light of the light emitting element, the optical sensor has the driving current of the light emitting element adjusted. The driving current of the light emitting element is adjusted such that the detected value of the image bearing member portion falls within the predetermined range. The optical sensor has the driving current of the light emitting element adjusted after a printing operation on a predetermined number of pages or on startup, to thereby suppress the effect of the change in amount of light of the light emitting element caused by the toner soil and the change with time on the measurement result.
A surface-mounted optical sensor, in which the light emitting element and the light receiving element are mounted on one substrate, can be downsized and reduced in cost. The light emitting element and the light receiving element for surface mounting are directly mounted on a surface of the substrate, and hence are difficult to adopt the structure of being encapsulated with a resin as in a round light emitting diode. Therefore, irradiation light from the light emitting element may be propagated on the surface of the substrate, and enter an inner layer of the substrate to reach the light receiving element. Such light reaching the light receiving element from the light emitting element is referred to as “leak light”. An amount of leak light varies for individual optical sensors due to variations in part tolerances and assembly accuracy.
When the optical sensor in which the leak light occurs is controlled such that the detected value, which is the measurement result, is kept constant within the predetermined range, the individual differences of the optical sensors do not occur in the detected value of the image bearing member portion. However, with the optical sensor in which the leak light occurs, when the image density of the measurement image is increased, the individual differences of the optical sensors occur in the detected value due to the effect of the amount of leak light. Therefore, the detected value has a high output offset and a large variation. Moreover, experiments have shown that the amount of leak light is increased when the driving current of the light emitting element is increased or an output gain of the light receiving element is increased to suppress the effect of the toner soil in addition to the individual differences of the optical sensors. Therefore, the optical sensor may be changed in characteristics with the increase in the amount of leak light. As described above, the optical sensor is increased in individual differences of sensitivity characteristics by the amount of leak light, and is changed in sensitivity characteristics every time the driving current of the light emitting element is adjusted or the output gain of the light receiving element is switched. The change in sensitivity characteristics of the optical sensor makes it difficult to accurately measure the image density, and hinders accurate density correction. As a result, quality of the image formed by the image forming apparatus is reduced. In view of the above, it is an object of the present invention to detect an image density with high accuracy while suppressing a change in sensitivity characteristics of an optical sensor, which is caused by leak light.